Recently, hydrogen energy attracts attention as clean fuel substituting for fossil fuel. As an example of application of hydrogen energy to motor vehicles, there is a fuel-cell electric vehicle which is driven by electric power generated by fuel cells mounted on it, wherein the fuel cells generate electric power by electrochemical reaction of hydrogen and oxygen in air. Hydrogen in gaseous state is stored in a high-pressure container which is mounted on the vehicle. Also, a hydrogen gas vehicle equipped with an internal combustion engine in which hydrogen gas from the high-pressure container storing the hydrogen gas is combusted is being developed.
Hydrogen is an explosive gas that is colorless and odorless. To use hydrogen more safely, a hydrogen gas station to supply motor vehicles with hydrogen gas must be provided with a set of functions to prevent explosive accidents, including detecting gas leakage quickly, notifying its manager of danger, and shutting off the gas at the main valve.
For conventional hydrogen gas stations, several sensors for detecting hydrogen gas have been installed per station.
Meanwhile, Japanese Patent Laid-Open No. H10(1998)-320675 encloses a wireless warning system that detects leakage of gas, electricity, or water at home and wirelessly transmits leakage information.
Japanese Patent Laid-Open No. H11(1999)-306463 encloses a gas leakage warning system oriented to city gas. This patent document describes a technique of setting up alternate routes to ensure a bandwidth for communications, that is, a system in which, when a wireless communication channel is occupied by a particular sensor and unavailable for other sensors, communication to an idle access point is attempted.
Since hydrogen is an explosive gas that is colorless and odorless, there are a number of problems to be solved for its practical use. For instance, to set up hydrogen gas stations at suitable places in an urban area, allowing motor vehicle users to get hydrogen gas supply with ease, it is needed to ensure high safety. Thus, it is conceivable to detect hydrogen leakage with high precision by installing dozens of sensors per hydrogen gas station.
In general, installing a great number of hydrogen sensors involves a problem of cost for cable placing and installation. As described in Japanese Patent Laid-Open No. H10(1998)-320675, wireless communication between sensors (with warning means) and an access point (with network control means) is favorable for reducing the cost.
However, wireless communication bandwidth is narrower than wired communication bandwidth and high reliability of wireless communication is hard to ensure. Because dozens of sensors are necessary per station, if hydrogen gas stations are set up in an urban area, a sufficient bandwidth must be allocated to a wireless communication channel to avoid channel overflow even in a case where simultaneous communication from all sensors occurs and this is considered to require a lot of cost.
A problem specific to hydrogen gas is that the diffusion speed of hydrogen gas is faster than that of other gasses. If hydrogen leakage occurs in a hydrogen gas station where a great number of sensors are installed and wirelessly connected, it is supposed that all sensors in the vicinity of the leakage location begin to transmit information of an abnormality to an access point all at once. Particularly because hydrogen is a gas with the smallest molecular weight, once it leaks out, it diffuses in a moment, faster than other gasses, and activates all sensors in the vicinity of the leakage location.
As a result, it is expected that congested access from sensors more than the allowable number may take place, causing the wireless channel to overflow. The channel overflow may give rise to the worst case where shut-off valves are not actuated due to failure of transmission of warning to the access point in spite of hydrogen leakage detected by the sensors.
A solution to this problem is allocating a bandwidth that allows for simultaneous communication from all sensors to the wireless channel in advance.
Also, it is conceivable to use the method of setting up access points for alternate routes in preparation for saturated communications, as described in Japanese Patent Laid-Open No. H11(1999)-306463. However, for a system like the hydrogen gas station where almost all sensors start to transmit a warning simultaneously when gas leakage occurs, a great number of access points for alternate routes are required and a very high cost is required to realize the system.